The present invention relates to the manufacture of optical waveguide fibers.
Optical waveguide fibers have been greatly improved during the last decade. Fibers exhibiting very low losses are generally formed by chemical vapor deposition (CVD) techniques which result in the formation of extremely pure materials. In accordance with these techniques, optical waveguide preforms can be formed by depositing glass layers on the outside surface of a temporary mandrel, or on the inside surface of a tube which later forms at least a portion of the cladding material, or by some combination of these techniques. These two embodiments of the CVD technique will be briefly described below.
In accordance with one embodiment of the CVD technique, often referred to as the inside vapor phase oxidation process, the reactant vapor, together with an oxidizing medium, flow through a hollow, cylindrical substrate. The substrate and the contained vapor mixture are heated by a source that moves relative to the substrate in a longitudinal direction, whereby a moving hot zone is established within the substrate tube. A suspension of particulate material which is produced within the hot zone travels downstream where at least a portion thereof comes to rest on the inner surface of the substrate where it is fused to form a continuous glassy deposit. After suitable layers have been deposited to serve as the cladding and/or core material of the resultant optical waveguide fiber, the temperature of the glass tube is generally increased to cause the tube to collapse. The resultant draw blank is then drawn in accordance with well known techniques to form an optical waveguide fiber having the desired diameter.
In another embodiment of the CVD process the vapor of reactant compounds is introduced into a flame where it is oxidized to form a glass particulate material or soot which is directed toward a mandrel. This so-called flame hydrolysis or outside vapor phase oxidation method of forming coatings of glass soot is described in greater detail in U.S. Pat. Nos. 3,737,292; 3,823,995; 3,884,550; 3,957,474 and 4,135,901. To form a step-index optical waveguide fiber, a second coating having a lower refractive index than the first is applied over the outside peripheral surface of the first coating. To form a gradient index fiber, a plurality of layers of glass soot are applied to the starting member, each layer having a progressively lower index of refraction as taught in U.S. Pat. No. 3,823,995. Gradient index fibers may also be provided with a coating of cladding material. After the plurality of coatings are formed on the mandrel, the mandrel is generally removed and the resultant tubular preform is gradually inserted into a consolidation furnace, the temperature of which is sufficient high to fuse the particles of glass soot and thereby consolidate the soot preform into a dense glass body in which no particle boundaries exist. In one embodiment of the outside vapor phase oxidation process, which is described in U.S. Pat. No. 3,957,474, the starting rod forms the core of the resultant fiber. The deposited cladding soot is consolidated on the surface of the core rod. The resultant consolidated blank is drawn into an optical waveguide fiber.
Although CVD techniques of forming optical waveguide preforms result in the formation of optical waveguide fibers having extremely low attenuation, they are relatively expensive. The size of preform which can be formed by the inside vapor phase oxidation process is relatively limited. The length of the hollow cylindrical substrate tube is limited to that length which can be supported between two separated chucks while being heated to reaction temperature. The substrate tube diameter is also limited in that process.
Fiber manufacturing cost can be lowered by increasing preform size or by continuously drawing fiber from a preform while the preform is being formed. Both of these cost reducing techniques decrease the number of preform handling and processing steps per unit of fiber length.
The outside vapor phase oxidation technique readily lends itself to cost reducing modifications. Initially, preforms were made larger by increasing the diameter. This was initially accomplished by traversing the burner longitudinally along the soot preform and adding thereto additional layers of increasing radius. Thereafter, axial techniques were developed whereby one or more burners or other soot depositing nozzles were directed axially toward a starting member. As the thickness of the deposited soot layer increases, the starting member moves away from the burners. Axial vapor phase oxidation techniques are taught in U.S. Pat. Nos. 3,966,446, 4,017,288, 4,135,901, 4,224,046 and 4,231,774.
A hybrid technique whereby a core is formed by axial vapor phase oxidation and a cladding layer is simultaneously deposited on the core by radially inwardly directed glass soot streams is taught in U.S. Pat. Nos. 3,957,474 and 4,062,665. As the core is formed, it is withdrawn from the burners or nozzles which formed it. The cladding is deposited by stationary burners or nozzles.
Substantially continuous methods of forming optical waveguide fibers by vapor phase oxidation techniques are taught in U.S. Pat. No. 4,230,472 issued to P. C. Schultz, U.K. Patent Application GB 2,023,127A and U.S. Pat. No. 4,310,339 issued to M. G. Blankenship.
In accordance with the Schultz patent a substantially continuous core member is longitudinally translated while there is simultaneously applied thereto an adherent coating of particulate material to form a continuous and substantially homogeneous adherent coating of substantially uniform thickness. The composite so formed is simultaneously or subsequently heated to sinter or consolidate the applied adherent coating thereby forming a solid blank which may be heated to the drawing temperature of the material thereof and drawn to reduce the cross-sectional area thereof, thereby forming a substantially continuous optical waveguide. The core member comprises the core while the consolidated coating comprises the cladding of the resultant optical waveguide. The adherent coating may be sintered or consolidated to form a solid blank and thereafter drawn in a separate operation or subsequently drawn as part of a continuing operation. As an alternative, the optical waveguide may be drawn immediately following the sintering or consolidation step employing a single heating of the structure.
In accordance with GB 2,023,127A a bare fiber core is drawn from a heated glass rod. Cladding is formed on the core fiber by vapor deposition of fine granules of glass which are thereafter heated to form a consolidated glass cladding.
The Blankenship patent teaches a substantially continuous method of forming an article suitable for an optical waveguide preform. The preform is formed by providing a starting member or bait and applying the particulate material to the exterior surface of the starting member to form a coating thereon. The coating is longitudinally translated while simultaneously additional particulate material is applied to the coating to form a preform body with the preform body thereafter being longitudinally translated. While longitudinally translating the preform body and applying additional particulate material to the end thereof, the starting member is continuously removed from the preform body leaving a longitudinal aperture remaining in the preform body. The so formed preform may thereafter be heated, consolidated, and drawn into an optical waveguide fiber.
An important and probably limiting factor in determining the deposition rate in the aforementioned CVD processes is related to the temperature of the gas stream in which the soot particles are entrained. See the publication, P. G. Simkins et al., "Thermophoresis: The Mass Transfer Mechanism in Modified Chemical Vapor Deposition", Journal of Applied Physics, Vol. 50, No. 9, September, 1979, pp. 5676-5681. Thermophoresis drives the soot particles from the hotter parts of the gas stream toward the cooler parts. Because the preform surface is usually cooler than the surrounding gas stream, the action of thermophoresis tends to drive the soot particles toward the preform surface. When a surface is nearly as hot as the surrounding gas steam, the temperature gradient is low. Thus, the thermophoresis effect is minimal, and the deposition rate is low. However, when the surface temperature of the preform is low, the thermophoresis effect due to the large thermal gradient results in a relatively high deposition rate.
In the aforementiond prior art, a burner is continuously directed at one position on the preform. Thus, the preform surface becomes hot, and the rate of deposition is limited by the small temperature gradient between the preform surface and the soot containing gas stream.